"Running is good for you if you do it right," says US exercise physiologist Kevin Jacobs, at the University of Miami.

"It builds your aerobic capacity, so your body is better able to use oxygen, which is good for you. It can improve blood lipids and blood glucose if they're abnormal. You sleep better. You feel better during the day."

But when you see sometimes waif-like marathon runners sweating and straining and vomiting and collapsing - sometimes even crawling across the finish line - the question arises of why they would put themselves through such misery.

Natalie Newton, a sports psychologist from Atlanta who specialises in long-distance runners, puts her academic spin on it.

"Any aerobic exercise releases endorphins, which make you feel exhilarated. A side benefit is that they help dull pain.

"Unfortunately, a lot of sports figures injure themselves and don't realise it until later."

Endorphins, opiate-like hormones sometimes called "the body's natural pain relievers", are released by the pituitary gland during strenuous aerobic exercise. Another way long-distance running is different from other sports, experts say, is that runners often peak into their 30s.

"The training involved and the duration of the event takes patience, and patience takes maturity," says Jacobs.